IN recent years a number of biosynthetic sequences have been shown to be regulated by feedback mechanisms, which have been of two general types, the first involving regulation of the activity of an enzyme by an inhibitory effect of an end-product1,2, the second involving regulation of the rate of synthesis of an enzyme by negative-induction, or suppression, of synthesis by an end-product3,4. In both types of mechanisms the inhibition involves the earliest step leading specifically to the synthesis of the regulating compound, which is itself a product normally present in the cell in appreciable concentration1,2.